1. Field of the Invention
This invention relates generally to production tubing for Moineau-type progressing cavity pumps, and, more specifically, to minimizing core deflection in the manufacture of stators for relatively long sections of production tubing and providing rotor bearing surfaces at periodic intervals in the manufactured production tubing.
2. Background
A progressing cavity pump is a positive displacement pump particulary adaptable for pumping viscous, abrasive or corrosive liquids. Rene J. L. Moineau is credited with conceiving the progressing cavity pump in 1932. Such pumps are occasionally referred to as single screw pumps. This name arises since the rotor of a typical progressing cavity pump is a single helix which rolls eccentrically in a stator having a cavity forming a double helix. This single helix rotor/double helix stator combination creates pockets which are moved (progressed) linearly from an inlet end to a discharge end of the pump as the rotor is turned.
Applications for progressing cavity pumps fall into two general categories, that is, metering or liquid transfer. Progressing cavity pumps function exceptionally well for metering purposes since they deliver a highly reliable predetermined quantity of liquid for each revolution of the pump rotor. By accurately governing the rate of revolution of the pump rotor, the quantity of liquid delivered by a progressing cavity pump can be accurately repeated. For this reason, progressing cavity pumps are frequently employed in chemical processing systems wherein accurate proportional blending or mixing of liquid components is required.
The other basic application for progressing cavity pumps is for liquid transfer using either a constant speed or a variable speed drive. These pumps adapt well to many speciality applications, such as handling abrasive, viscous and two-phase fluids. Progressing cavity pumps can be employed for pumping fluids having a viscosity less than one centipoise.
The progressing cavity pump rotor is generally configured with a single screw thread of streamlined design without sharp edges functioning inside the cavity of the stator, the interior wall of which defines the elongated double helix. The rotor and stator are positioned within a length of production tubing (the pump barrel), the rotor being driven by a drive shaft connected to a motor. While the stator can be formed of metal, the most common method of manufacturing progressing cavity pumps is to make the stator of an elastomeric material. The combination of a metallic rotor and an elastomeric stator functions advantageously to provide a pump having great capacity to pump abrasive fluids and to maintain a predetermined discharge pressure.
Typically, the maximum pressure that a progressing cavity pump can deliver is directly related to the length of the rotor and stator, and, accordingly, in some applications, such as downhole submersible pumps for the oil field, the rotor and stator can be relatively long compared to their diameters.
Conventionally, the manufacture of production tubing involves vertically supporting a length of tubular casing on an injection plate and fixing a core centrally within the casing coincident to the longitudinal axis of the casing. Elastomeric material is then injected under very high pressure into the casing, the inside surface of which has been treated with a bonding material. The elastomeric material flows upward around the core to form the stator after curing.
A significant problem in the manufacture of relatively long stators is that the core tends to deflect within the tubular casing during the injecting step. The core is prone to move to one side or the other in response to the very high pressure of the injected elastomeric material. Moreover, if a horizontal injection process is used, the core may deflect or bow due to its own weight. Core deflection results in crooked stators and larger tolerances which, in turn, detrimentally affect pump performance.
Prior attempts to correct the problem of core deflection include drilling and tapping a hole in the casing and inserting set screws against the core to maintain its position during the molding process or using a sliding "pig" or disk, essentially a ring, to ride on top of the column of injected elastomeric material to assist the core in maintaining a central alignment. These methods, however, have not achieved great success in preventing core deflection and have significant drawbacks, such as tooling damage caused by the set screws, wobbling or sticking of the sliding ring during the injection process, and the need to recover the ring after manufacture. For these reasons, there remains a need for efficiently and reliably centering the core within a long length of casing during stator formation.
Another problem regularly encountered in long lengths of production tubing involves the misalignment of the rotor within the stator cavity. Preferably there should be an equal interference between the rotor and stator around the entire circumference of the rotor to ensure optimum pump performance. Should the rotor wobble or "chunk out" to one side of the stator cavity the pump loses the beneficial equal interference and performance is degraded. If areas of significant interference are created by the misalignment the torques required to operate the pump increase to the detriment of the system.